Interpretive Summary: Inositol is widely distributed in plants and animals tissues as a structural component of cell membranes. Nutritional studies suggested that fish can synthesize inositol. Some fish species, however, cannot synthesize this vitamin in quantity sufficient to meet their metabolic needs, and thus, require an exogenous source of inositol in their diets. Deficiency signs of inositol reported in fish include poor growth, loss of appetite, fin erosion, dark skin coloration, slow gastric emptying and lipotropic. A possible role of inositol in the prevention and treatment of certain diseases of terrestrial animals has been reported. However, the essentiality of inositol in the diets of Nile tilapia is unknown. Thus, this study was conducted to evaluate the effect of dietary levels of inositol on growth performance, feed utilization, chemical composition, immune response and resistance of Nile tilapia to a bacterium, Streptococcus iniae. Results of the 10-week feeding study showed that supplementation of inositol to purified diets had no effects on growth, feed consumption and efficiency, hematological parameters, immune response and resistance of Nile tilapia to bacterial infection. However, supplementation of this vitamin at levels of 100 to 400 mg/kg diet was needed to prevent tissue lipid accumulation. Thus, Nile tilapia can synthesize inositol in amounts sufficient to meet the requirements for optimum growth, feed efficiency, immune function and disease resistance but insufficient to prevent tissue lipid accumulation. However, because inositol is widely distributed in common feedstuffs, practical feeds should contain adequate levels of this vitamin to meet various metabolic needs of Nile tilapia.

Technical Abstract:
A study was undertaken to determine the influence of dietary levels of inositol on growth performance, feed utilization, immune response and resistance of Nile tilapia (Oreochromis niloticus) to Streptococcus iniae challenge. Six isonitrogenous (38% crude protein), isocaloric (3,300 kcal DE/kg) purified diets supplemented with graded levels of inositol (0.0, 50, 100, 200, 400 or 800 mg/kg diet) were fed to juvenile tilapia (initial body weight of 5.8 ± 0.1 g) in triplicate aquaria twice daily to apparent satiation for 10 weeks. Supplementation of dietary inositol had no affect on final weight gain, feed intake, feed efficiency ratio and survival. Also, no significant differences were observed in total cell, red and white blood cell counts, hematocrit and plasma hemoglobin content among treatments. Hepatosomatic index was not significantly affected by dietary treatments. Fish fed the 800-mg inositol diet had significantly lower muscle lipid content than the groups fed diets containing 200 mg inositol/kg or lower but these values did not differ from that of fish fed the 400-mg inositol diet. Liver lipid content significantly decreased when dietary inositol was increased to 100 mg/kg diet or higher. Plasma lysozyme activity and antibody titers to S. iniae were not affected by supplementation of dietary inositol. Cumulative mortality of fish 14-day post challenge with S. iniae was likewise not affected by the dietary inositol levels. The results of this study indicate that Nile tilapia juveniles did not require an exogenous source of inositol for normal growth, feed utilization and erythropoiesis. Supplementation of dietary inositol had no effect on improving the resistance of tilapia to S. iniae infection. However, supplementation of inositol to purified diets is required to prevent lipotropic effect of inositol deficiency.